An attenuator circuit is a network used to reduce the power level of a signal without introducing any appreciable distortion. As used in conventional monolithic microwave integrated circuits (MMIC's), attenuator circuits are used to decrease the magnitude of radio frequency signals. Attenuator circuits, conventionally known as attenuators can be active or passive devices. Attenuators are well known in the art. See H. Kondoh, "DC-50 GHz MMIC Variable Attenuator with 30 dB Dynamic Range," IEEE MTT-S Digest, 1988; G. B. Norris, et. al, "A Fully Monolithic 4-18 GHz Digital Vector Modulator," IEEE MTT-S Digest, 1990, pp. 789-792; Y. Hwang, et. al, "A Microwave Phase and Gain Controller with Segmented-Dual Gate MESFETs in GaAs MMIC's," IEEE 1984 Microwave and MM-Wave Monolithic Circuits Symposium, 1984, pp. 1-5; R. Gupta, et. al, "A 0.05 to 14 GHz MMIC 5-bit Digital Attenuator," Proc. GaAs IC Symposium Digest, 1987, pp. 231-234; McGrath and Pratt, "An Ultra-Broadband DC-12 GHz 4 -bit GaAs Monolithic Digital Attenuator," Proc. GaAs IC Symposium Digest, 1991, pp. 247-250; and F. Ali, et. al, "Low-Loss, High-Power, Broadband GaAs MMIC Multi-bit Digital Attenuators with On-Chip TTL Drivers," Proc. GaAs IC Symposium Digest, 1991, pp. 243-246. Analog variable attenuators are able to achieve broadband performance. However, linearization for attenuation v. control voltage is required for such devices, and performance can vary according to the operating temperature.
Conventional digital attenuators may include the following circuit topologies to achieve broadband performance: segmented dual-gate active attenuators; binary weighted attenuators connected by transfer switches; and field-effect transistor (FET) based "non-distributed" attenuators. The disadvantages of these designs include: low attenuation accuracy and die-to-die variation for the dual-gate active attenuators; high insertion losses for the transfer switch approach; and limited frequency and attenuation range for the "non-distributed" attenuators.
It is an object of this invention to provide a digital attenuator circuit which employs a "distributed" topology implemented as an MMIC, wherein the impedance of the shunt elements of the circuit are matched with high-impedance transmission lines, to achieve broadband performance without any of the aforementioned limitations found in conventional attenuators.